Usually, two electrical machines are used in motor vehicles, a generator, which is driven by the engine of the motor vehicle and which generates electrical energy for supplying the electrical-system consumers and for charging the battery, and a starter, which works as a battery-powered electrical motor and which, in the event of a starting operation, brings the engine of the motor vehicle up to a required minimum rotational speed. Also known, however, are systems featuring a single electrical machine, which takes the form of a starter-generator and which accomplishes both the starting of the engine as well as the generation of the electrical energy.
Such starter-generators are, for example, flywheel starter-generators or crankshaft starter-generators directly connected to the crankshaft of the engine. The starter-generators used in this context are three-phase machines, which can be connected to the vehicle battery via a power electronics that includes a controlled rectifier bridge, in particular a pulse-controlled a.c. converter with a pulse-controlled inverter bridge. The pulse-controlled inverters are controlled using a control electronics.
If the electrical machine, for example, is a permanent-field synchronous machine, then the rotating machine will generate a voltage regardless of whether the electronics is activated or not. In the case of sufficiently high rotational speeds of the synchronous machines this can lead to an exceeding of the permitted voltage and there is the danger of destroying the electronics of the starter-generator as well as the electrical system electronics.
A vehicle electrical system having a starter-generator as well as a method for the automatic control of such a starter-generator, which can be connected to the vehicle battery via a pulse-controlled inverter bridge, is described in German Published Patent Application No. 197 33 212 for example. In this known system, the pulse-controlled inverter elements of the converter bridge are suitably controlled with the aid of a control electronics in such a way that an optimal generator control is achieved in generator operation and hence a maximum of electrical power is generated. At the same time, a suitable control of the pulse-controlled inverter elements ensures that in a starting operation the electrical machine can draw electrical power from the battery and thereby working as a motor can accelerate the vehicle engine to the rotational speed required for starting.
Since it is possible that the vehicle battery is destroyed during the operation of the vehicle or that the battery is largely drained due to unfavorable circumstances, there is the danger that in the resulting so-called batteryless operation, the electronics of the electrical machine is no longer supplied. In this case, when the machine is short-circuited and rotating, there exists no intermediate circuit voltage which could supply the control electronics. Hence there is the danger that, if the control electronics is not supplied, the short-circuiting of the machine cannot be maintained. The consequence would be an uncontrolled rise of the voltage. This could lead to destruction in the electronics of the electrical machine or of the starter-generator and in the vehicle electrical system itself.